1. Field of the Invention
The present invention relates to a light wavelength conversion module. More precisely, the present invention relates to a light wavelength conversion module which includes a semiconductor laser having an external resonator provided with a wavelength selecting element, and a light wavelength conversion element for converting a laser beam emitted from the semiconductor laser to a second harmonic wave or the like.
2. Description of the Related Art
Conventionally, various types of light wavelength conversion devices which convert a laser beam emitted from a semiconductor laser to a second harmonic wave or the like have been proposed, and have been used as a blue laser light source and/or a green laser light source. For example, a light wavelength conversion module is disclosed in Japanese Patent Laid-Open (JP-A) No. 10-254001. The light wavelength conversion module illustrated in FIG. 9 in this publication (JP-A No. 10-254001) includes a semiconductor laser which is provided with an external resonator and a wavelength selecting element such as a narrow band-pass filter or the like provided in the external resonator, and a light wavelength conversion element which is composed of a waveguide type second harmonic generation (SHG) element having a periodic domain reversing structure, wherein the semiconductor laser and the light wavelength conversion element are optically coupled directly with each other. In the light wavelength conversion module, a wavelength can be locked to a central transmitted wavelength of the narrow band-pass filter provided in the external resonator, and an oscillation wavelength of the semiconductor laser can be locked to a certain wavelength corresponding to a rotation angle of the narrow band-pass filter by rotating the filter.
A general semiconductor laser can oscillate a laser beam even without an external resonator since it has a resonator structure provided in an element thereof. However, the oscillation wavelength of the semiconductor laser prior to the locking of the wavelength fluctuates within a range of a few nanometers, and shifts toward the longer wavelength side as the driving current increases. For example, in a case in which the electric current is changed from 50 to 200 mA when a semiconductor laser having several longitudinal modes at intervals of about 0.2 nm is used, the central oscillation wavelength shifts about 5 nm toward the longer wavelength side due to heat generation of the semiconductor laser itself, as shown in FIG. 9 of the present application.
Therefore, when a semiconductor laser is optically coupled with an SHG element without locking the wavelength, the oscillation wavelength of the semiconductor laser does not coincide with a wavelength at which the wavelength conversion efficiency of the SHG element is maximized, i.e., does not coincide with a wavelength which phase-matches with the SHG element. The output light amount of the second harmonic wave fluctuates, resulting in almost no output of second harmonic waves. In order to solve this problem, in the light wavelength conversion module disclosed in JP-A No. 10-254001, an external resonator is provided, and an oscillation wavelength of the semiconductor laser is locked to a wavelength which phase-matches with the SHG element to thereby stabilize the outputted light amount of the second harmonic wave light.
However, even if the above-described locking of the wavelength is carried out, there still exists the following problem. The output light amount of the semiconductor laser itself increases linearly as the driving current of the semiconductor laser increases as shown in FIG. 10A when a threshold current (Iop) is exceeded. In contrast, the output light amount of the SHG element does not increase monotonically as the driving current of the semiconductor laser increases, but increases while repeatedly increasing and decreasing as shown in FIG. 10B, when the same semiconductor laser and SHG element are optically coupled to generate a second harmonic wave. That is, the IL characteristic (current vs. output characteristic) which expresses the relationship between the driving current of the semiconductor laser and the output light amount of the SHG element repeatedly increases and decreases.
When such increasing and decreasing of the output light amount occurs, there is a problem in that automatic power control (APC) for stabilizing the output light amount of the SHG element cannot be carried out properly when used. Moreover, there is another problem in that it is difficult to control the output light amount to a desirable amount when the output light of the SHG element is modulated by increasing and decreasing the driving current, since the output light amount of the SHG element does not increase monotonically as the driving current of the semiconductor laser increases.
The present invention is provided so as to solve the aforementioned problems, and an object of the present invention is to provide a light wavelength conversion module in which the output light amount of a light wavelength conversion element increases monotonically as the driving current of a semiconductor laser increases.
In order to solve the aforementioned problems, a first aspect of the present invention is a light wavelength conversion module including: (a) a light wavelength conversion element having a wavelength band, which when the light wavelength conversion element receives light within the wavelength band, emits light having a different wavelength; and (b) a semiconductor laser having an external resonator provided with a wavelength selecting element, the semiconductor laser being disposed for communicating light to the light wavelength conversion element and operable for producing light of a fundamental wavelength including a plurality of longitudinal mode spectra within the wavelength band of the light wavelength conversion element.
A second aspect of the present invention is a light wavelength conversion module including: (a) a light wavelength conversion element having a wavelength band, which when the light wavelength conversion element receives light within the wavelength band, emits light having a different wavelength; and (b) a semiconductor laser having opposite emitting end surfaces and an external resonator, the semiconductor laser being operable for producing light of a fundamental wavelength including a plurality of longitudinal mode spectra within the wavelength band of the light wavelength conversion element, which is disposed for receiving light from one emitting end surface of the semiconductor laser, and the external resonator being disposed for receiving light from the other emitting end surface, the external resonator including a wavelength selecting element and a reflecting member disposed on an optical path for receiving the light, with the reflecting member disposed on the optical path opposite the wavelength selecting element from the semiconductor laser.
A third aspect of the present invention is a light wavelength conversion module according to either of the first and second aspects, wherein the wavelength band has a length xcex94xcex, and a wavelength interval of the longitudinal mode spectra of the semiconductor laser is xcex94xcexm, and the number of the longitudinal mode spectra is Nmax, which is an integer part of the quotient of xcex94xcex/xcex94xcexm or less.
A fourth aspect of the present invention is a light wavelength conversion module according to either of the first and second aspects, wherein the light wavelength conversion element and the semiconductor laser are optically coupled directly to each other.
A fifth aspect of the present invention is a light wavelength conversion module according to either of the first and second aspects, wherein the light wavelength conversion element is a quasi-phase matching type light wavelength conversion element which performs wavelength conversion by quasi-phase matching.
A sixth aspect of the present invention is a light wavelength conversion module according to either of the first and second aspects, further including a driving device for driving the semiconductor laser in a modulated state according to a modulation signal.
A seventh aspect of the present invention is a light wavelength conversion module according to either of the first and second aspects, further including a driving device for driving the semiconductor laser at high frequency.
An eighth aspect of the present invention is a light wavelength conversion module according to the seventh aspect, further including another driving device for driving the semiconductor laser in a modulated state at a frequency less than the high frequency.
A ninth aspect of the present invention is a light wavelength conversion module according to either of the first and second aspects, wherein the wavelength selecting element has a light transmission half-width through which the plurality of longitudinal mode spectra of the light of the fundamental wavelength is transmissible.
A tenth aspect of the present invention is a light wavelength conversion module according to the ninth aspect, wherein the half-width of the wavelength selecting element is 0.5 nm or more.
An eleventh aspect of the present invention is a light wavelength conversion module according to either of the first and second aspects, wherein the semiconductor laser has opposite ends, and a reflection reducing coating having a reflectance of 20% or more is provided on at least one end surface of the semiconductor laser.
A twelfth aspect of the present invention is a light wavelength conversion module according to either of the first and second aspects, wherein the light wavelength conversion element includes a light waveguide formed by a proton exchange annealing process.
A thirteenth aspect of the present invention is a light wavelength conversion module according to either of the first and second aspects, wherein the light wavelength conversion element includes an optical crystal base formed of LiNbO3 doped with MgO or ZnO, or formed of LiTaO3 doped with MgO or ZnO.
In order to investigate the causes of the above-described increase and decrease in the output light amount of the SHG element, the present inventors utilized the optical system, which is shown in FIG. 11 and is formed by a semiconductor laser provided with an external resonator for locking wavelengths, and let an oscillation spectrum of the semiconductor laser 110, in a state in which the wavelength thereof was locked, pass through an optical fiber 112 and magnified the range by an optical spectrum analyzer 114 to observe the oscillation spectrum of the semiconductor laser 110. In FIG. 11, reference numeral 116 indicates an external mirror which forms the external resonator, reference numerals 118, 120 and 122 indicate lenses, and reference numeral 124 indicates a band-pass filter. A semiconductor laser, which had an oscillation wavelength of 950 nm and had a light output of 70 mW when the laser was driven by a 200 mA current, and in which the external resonator had a length of 750 xcexcm and the input and output end surfaces had a reflectance of 20 to 30%, was used as the semiconductor laser 110. A dielectric multi-layer film reflecting mirror having a reflectance of 99% was used as the mirror 116. Lenses having a numerical aperture of 0.5 were used as the lenses 118, 120 and 122. A band-pass filter having a half-width of transmitted light of 0.5 nm and a transmittance of the central wavelength of 80% was used as the band-pass filter 124.
According to the observations of the present inventors, the oscillation wavelength of the semiconductor laser repeatedly fluctuated within a width of about 0.2 nm in the vicinity of a central wavelength of waves transmitted through the band-pass filter. More precisely, as shown in FIG. 12, as the driving current increases, the oscillation wavelength gradually moves within a range of transmitted wavelengths of the band-pass filter from a shorter wavelength side to a longer wavelength side, and when it reaches the right end (the longer wavelength side), the oscillation wavelength hops to the left end (the shorter wavelength side). This hopping of the oscillation wavelength is repeated. It is assumed that when a second harmonic wave is generated by optically coupling the semiconductor laser with the SHG element, the IL characteristic repeatedly increases and decreases due to this wavelength hop.
According to the studies of the present inventors, it is believed that the aforementioned wavelength hop is caused by the following phenomena. In a semiconductor laser, the both end surfaces of the laser element oscillate a laser beam as a resonator, so that several spectra of the oscillation wavelength are observed. This oscillation is oscillation in the Fabry-Pxc3xa9rot mode (FP mode) of the semiconductor laser, and oscillation with two or more spectra is referred to as oscillation in which the so-called longitudinal mode is a multi-mode. In a case in which the longitudinal mode is a multi-mode, when locking of the wavelength is carried out by using the above-described external resonator, laser oscillation occurs only when the oscillation wavelength by the FP mode coincides with a central transmitted wavelength having the highest transmittance of the band-pass filter, thereby locking the wavelength.
On the other hand, the FP mode of the semiconductor laser shifts gradually toward the longer wavelength side due to the generated heat as the driving current increases. Therefore, even in the wavelength locked state, the FP mode of the semiconductor laser shifts minutely within the range of transmitted wavelengths of the band-pass filter. When a single FP mode moves toward the longer wavelength side as described above and the transmittance of the band-pass filter with respect to the single FP mode deteriorates such that the oscillation mode is stopped, the next FP mode adjacent to the previous FP mode at the shorter wavelength side thereof enters into the range of transmitted wavelengths of the band-pass filter and this FP mode oscillates the laser beam. Accordingly, it seems that, as the driving current increases and decreases, the oscillation wavelength repeats hopping with an interval (0.2 nm in the above-described example) coinciding with the FP mode interval of the semiconductor laser.
In accordance with the present invention, a semiconductor laser, which includes an external resonator provided with a wavelength selecting element, emits a fundamental wave including a plurality of longitudinal mode spectra within a range of an acceptable wavelength band of a light wavelength conversion element. Thus, even if a wavelength hop occurs in any oscillation spectrum, a wavelength hop does not occur in other oscillation spectra, thereby enabling oscillation with a relatively stable wavelength. Accordingly, when a wavelength conversion to a second harmonic wave or the like is carried out by optically coupling the semiconductor laser and the light wavelength conversion element such as an SHG element, the IL characteristic varies monotonically. That is, the output light amount of the light wavelength conversion element increases monotonically as the driving current of the semiconductor laser increases.
Moreover, the number of the longitudinal mode spectra is determined such that the spectra exist within the acceptable wavelength band of the wavelength conversion element. However, as the number increases, the power of the wavelength-converted wave such as the second harmonic wave deteriorates. Thus, the number of the longitudinal mode spectra is preferably at most Nmax, which is the integer part of the quotient of xcex94xcex/xcex94xcexm, where xcex94xcex is the acceptable wavelength band of the light wavelength conversion element and xcex94xcexm is the wavelength interval of the longitudinal mode spectra of the semiconductor laser. In the formula, xcex94xcex is a wavelength band in which the output of the light whose wavelength has been converted by the light wavelength conversion element is one-half of the maximum value, i.e., a half-width.
For example, in a case in which a module is formed by optically coupling a semiconductor laser of 950 nm with an SHG element having a periodic domain reversing structure with a period of 4.7 xcexcm, an acceptable wavelength band xcex94xcex of the SHG element varies depending on a periodic reversal length Lc of the SHG element. When Lc is 10 mm, xcex94xcex is 0.11 nm, and when Lc is 1 mm, xcex94xcex is 1.2 nm. However, in order to obtain a practical output light amount (0.1 mW or more) of the SHG element, the periodic reversal length Lc of the SHG element needs to be 1 mm or more, and the maximum value of xcex94xcex at this time is 1.2 nm. Assuming that a wavelength interval xcex94xcexm of the longitudinal mode spectra of the semiconductor laser is 0.2 nm, Nmax is 6. That is, in this case, up to six longitudinal mode spectra are acceptable. In this way, the value of xcex94xcex can be determined by the target output light amount of the SHG element, and the value of Nmax can be obtained properly in accordance with the determined value of xcex94xcex.
There are several methods for causing a semiconductor laser having an external resonator provided with a wavelength selecting element to emit a fundamental wave including a plurality of longitudinal mode spectra within an acceptable wavelength band of a light wavelength conversion element. Such methods include, for example, 1) driving the semiconductor laser at a high frequency; 2) setting the half-width of transmitted light of the wavelength selecting element to a width through which a plurality of longitudinal mode spectra included in the fundamental wave emitted from the semiconductor laser can be transmitted; and 3) providing a reflection reducing coating having a reflectance of 20% or more on at least one of end surfaces of the semiconductor laser.
The longitudinal mode spectrum in accordance with the present invention means a spectrum which can be decomposed when it is measured by an optical spectrum analyzer having a resolving power of about 0.1 nm.